Wireless terminals, such as mobile phone handsets, typically incorporate either an external antenna, such as a normal mode helix or meander line antenna, or an internal antenna, such as a Planar Inverted-F Antenna (PIFA) or similar.
Such antennas are small (relative to a wavelength) and therefore, owing to the fundamental limits of small antennas, narrowband. However, cellular radio communication systems typically have a fractional bandwidth of 10% or more. To achieve such a bandwidth from a PIFA for example requires a considerable volume, there being a direct relationship between the bandwidth of a patch antenna and its volume, but such a volume is not readily available with the current trends towards small handsets. Further, PIFAs become reactive at resonance as the patch height is increased, which is necessary to improve bandwidth.
U.S. Pat. No. 6,061,024 discloses a duplexing antenna for a single band, for example 800 to 900 MHz, portable radio transceiver in which the antenna comprises respective PIFA transmit and receive antennas formed as patches on a printed circuit board mounted above and facing a reference ground plane of a circuit board on which the transmitter and receiver components are mounted. Separate feeds interconnect an output bandpass filter of the transmitter and an input bandpass filter of the receiver with their respective patch antenna. An electrically conductive pedestal connects the reference ground plane to an elongate area of the printed circuit extending between the patches. Both the transmit and receive antennas are narrow band, say 1.6 MHz, antennas which are tunable over a wider bandwidth, say 25 MHz, by coupling reactive components, that is capacitances or inductances, to the respective antennas using PIN diode switches.
Our pending unpublished PCT Patent Application IB02/05031 (Applicant's reference PHGB 010194) discloses a wireless terminal having a dual band PIFA comprising a substantially planar patch conductor. A first feed conductor comprises a first feed pin connected to the patch conductor at a first point, a second feed conductor comprises a second feed pin connected to the patch conductor at a second point, and a ground conductor comprises a ground pin connected between a third point on the patch conductor and a ground plane. The feed and ground pins may have different cross-sectional areas to provide an impedance transformation. First and second transmission lines are formed by the ground conductor and a respective one of the feed conductors. The first and second transmission lines are short circuit transmission lines whose respective lengths are defined by a first linking conductor connecting the first feed and ground pins and a second linking conductor connecting the second feed and ground pins. Complementary circuit elements comprising first and second shunt capacitance means are coupled respectively between the first and second feed pins and the ground pin. The described antenna is fed by a diplexer to provide isolation between say GSM circuitry operating over a frequency band 880 to 960 MHz and DCS circuitry operating over a frequency band of 1710 to 1880 MHz. The provision of a diplexer although enabling the cited antenna arrangement to work satisfactorily represents an undesired complication.